Influence of synthesis parameters on the performance of CeO2–CuO and CeO2–ZrO2–CuO systems in the catalytic oxidation of CO in excess of hydrogen
Graphical abstract
TEM images of the sample FCZCu, a Ce–Zr–Cu oxide system with flower like morphology, prepared by slow co-precipitation in the absence of any structure directing agent.
Highlights
► Nanostructured Ce/Cu and Ce/Zr/Cu oxide systems were prepared. ► The effect of the different morphology and of the role of Zr doping on the active species was evaluated. ► The CO-PROX catalytic activity was correlated with the structural characteristics.
Introduction
A multi-component heterogeneous catalyst is usually the association of one or more transition metals with one or more oxides, the activity and selectivity of which is determined by numerous factors. Nevertheless the complete control of the design and synthesis of these systems is still quite far from being achieved and the usual synthetic methods give often difficult to characterize catalytic materials. In fact the catalytic properties of a material depend not only on the chemical nature of the components and their ratio but also on the particles dimension and even on their nano-or micro morphology, all strongly influenced by the preparation method. [1], [2]
In the previous years the demand has grown for environment benign catalytic systems related to low-temperature renewable energy production. Catalytic CO oxidation with oxygen in the presence of an excess of H2 (CO-PROX), is one of the most economical and efficient approaches to reduce the CO content of the H2-rich gas streams produced by reforming of alcohols or hydrocarbons down to ppm level [3].
Some years ago, Liu and Flytzani-Stephanopoulos [4] reported the extraordinary activity of the CuO–CeO2 system in the elimination of CO at relatively low temperatures. Now, it is known that the high performances of CuO–CeO2 based catalysts are attributable to the strong interaction between highly dispersed copper species and the ceria surface, which favors the formation of oxygen vacancies at the copper–ceria boundaries, thus increasing the Cu reducibility [5], [6], [7], [8], [9]. It is also known that addition of Zr to ceria modifies the redox properties, the oxygen-storage capacity and thermal resistance of the latter [8], [10], [11], [12], [13], [14], [15], [16].
The performance of these three-component catalysts, in comparison with CuO–CeO2 and CuO–ZrO2 binary systems, was found strongly dependent not only from the molar ratio among reagents but also from the preparation methodology, that can influence the morphological and structural characteristics of the materials as well as the interaction between the oxides and the dispersion of the active phase [17], [18], [19], [20], [21], [22]. It was also found that the catalytic behavior of nanostructured ceria, can be significantly influenced by the sample morphology [23], [24], [25], [26], [27].
Nanostructures with a 3-dimensional flower-like morphology have been described for various oxides: a flower-like Ce1−xZrxO2 solid solution was electrochemically grown [28], well formed flower-like nano-architectures of CuxO [29], [30], ZnO [31], [32], [33], Fe2O3 [34], Co3O4 [35], MnO2 [36] and La, Pr doped CeO2 [37] have been prepared by slow hydrothermal processes.
In a previous paper [38], we described the preparation and CO-PROX catalytic activity of a nanostructured Ce/Zr/Cu oxide system with a flower-like morphology. Before catalytic testing, the sample was thermally pre-treated at various temperatures in the 350–650 °C range and it was found that the thermal treatment induces only slight structural changes without altering the overall morphology. The sample treated at 650 °C showed the better CO-PROX activity.
In this work we report the preparation of Ce–Cu and Ce–Zr–Cu mixed oxide systems by slow co-precipitation in the absence of any structure directing agent and by a traditional co-precipitation method. The catalytic performance in the CO Preferential Oxidation of the four obtained samples, previously pretreated at 650 °C, was tested.
Section snippets
Reagents
All the materials used are Aldrich products and no further purification was carried out.
Method (1): Slow co-precipitation
A 0.30 M aqueous solution of K2CO3 was slowly added (0.4 mL min−1) to an aqueous solution of CuCl2·2H2O (1.4 mmol) and CeCl3·7 H2O (8.1 mmol) (in the case of CeCu) and to an aqueous solution of CuCl2·2H2O (1.4 mmol), CeCl3·7H2O (8.1 mmol) and ZrOCl2·8H2O (0.9 mmol) (in the case of CeZrCu) under stirring, until a pH value of 8.2 was reached. The suspension was allowed to settle for 20 h, then centrifuged. The
Scanning (SEM) and transmission (TEM) electron microscopies
The surface morphology of the obtained samples was investigated by scanning electron microscopy (SEM) (Fig. 1a–d). A general view of the materials revealed that both samples prepared by slow precipitation, FCCu and FCZCu, present a flower-like morphology, made up by micro-sheets with quite homogeneous size and shape (Fig. 1a and c). The petals are about 200 nm thick and about 10 μm long.
The exact mechanism for the formation of these petals is still unclear, but it is generally suggested that,
Acknowledgements
The authors wish to thank Martina Marchiori (Ca’ Foscari University – Venice) for N2 physisorption and TPR-TPO measurements and Dr. Davide Cristofori (Ca’Foscari University-Venice) for the HRTEM measurements.
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